Long life x-ray source target

ABSTRACT

Disclosed herein is an X-ray lithography system having a long life target. The life of the conventional target in X-ray generating systems for use in X-ray lithography systems is increased by providing means by which a single laser pulse can be provided to the same spot a plurality of times. In addition, new target designs are provided which are mechanically moved to allow laser pulses to be provided to adjacent points over a large surface area. One type of target is a cylindrical drum which is helically rotated to allow the laser pulse to intersect at all points along the helix of the drum. A second type of long life target is a long continuous strip in which a strip is moved from a feed reel to a take-up reel. The strip may be within a cassette.

This invention relates to X-ray targets and more particularly to a longlife target which is used in conjunction with an excitation beam toproduce a plasma which emits soft X-rays for use in a commercial X-raylithography system.

X-ray lithography systems are well known in the prior art. They werefirst suggested by Smith et al. in U.S. Pat. No. 3,743,842. As describedin this patent, a source of X-rays was generated by directing anelectron beam against a target placed in an evacuated chamber. The beamparticles are of sufficient energy so that X-ray emission is excited atthe point where the beam strikes the target. The emitted X-rays, inturn, are directed towards an X-ray resist covered semiconductorsubstrate to expose the resist. If an X-ray mask is placed between thetarget and the semiconductor, a patterned exposure can be formed on theresist coated substrate.

The X-ray lithography system of Smith was taken one step further asdescribed in U.S. Pat. No. 4,184,078 to Nagel, et al. In this patent theelectron beam is replaced by a laser beam pulse, which is directedtowards a target to create the plasma which emits the X-rays. In thelaser generated X-ray system, each time a laser pulse strikes thetarget, the temperature of the target material is raised to severalmillion degrees, thereby forming the plasma. During the formation of theplasma, a certain amount of the target material is burnt away, leaving asmall cavity in the target. In the prior art, when people have attemptedto create X-rays by directing a subsequent laser pulse into the formedcavity, it had been determined that the intensity of the X-raysgenerated was substantially smaller. Thus, each time a new burst ofX-rays was desired, either the target was moved so that the laserimpinged on an unused flat area of the target or a new target wasinserted. In either case, the apparatus described in the prior art isnot useful in commercial applications because of the long time periodsrequired to move the target from place to place or to replace the targetwhile in the evacuated chamber. In present ultra-violet lithographysystems, a semiconductor wafer is moved to the exposure area every oneor two seconds. For X-ray lithography to be useful, it is necessary toprovide a new laser shot to cause an X-ray plasma to be created eachtime the wafer is in position, which should be no less often than thesame one or two seconds.

A significant additional problem in the prior art techniques has beenthe generation of a sufficient intensity of X-rays to actually exposethe X-ray resist coated over the semiconductor wafer. In past techniquesthe power of the laser applied to the target was so small that it wasnecessary to fire the laser ten or more times in order to obtain asufficient number of X-rays to totally expose the target. It is wellknown that increasing the power of the laser pulse will create a hotterplasma which, in turn, generates X-rays of a greater intensity. Howeverpractical limits may prevent delivery of a sufficiently powerful laserpulse.

One can also select different materials for the target to provide themaximum intensity X-rays. However, the X-rays may be desired to be at acertain wavelength and this will limit the choice of materials.

It would be preferable to have techniques available which will increasethe intensity of the X-rays emitted from the plasma through the use ofexisting laser sources and target material.

In accordance with one aspect of this invention, there is provided anX-ray generating system comprising an excitation beam provided along apath and a target having a cavity therein. The target is positioned sothat the cavity intersects the path and the beam is of sufficient powerto cause an X-ray emitting plasma to be created at least partiallywithin the cavity. In addition the system includes an object to beradiated by the emitted X-rays positioned within a certain angle of thepath. The certain angle is that angle in which the intensity of theprovided X-rays is greater than the intensity of the X-rays provided ifa part of the target other than the cavity was positioned to intersectthe path.

One preferred embodiment of the subject invention is hereafter describedwith specific reference being made to the following Figures, in which:

FIG. 1 shows an X-ray lithography system using a fixed target;

FIGS. 2A, 2B, 2C and 2D show the formation of a cavity in the targetafter several applications of a laser pulse to the target;

FIG. 3 shows an X-ray lithography system having a first type of movabletarget mechanism; and

FIG. 4 shows an X-ray lithography system having a second type of movabletarget mechanism.

Referring now to FIG. 1, an X-ray lithography system 10 is shown whichincludes the laser generator 12 which generates a high powered pulselaser beam 14. Laser beam 14 is focused by a lens 16 onto a target 18.Target 18 may be of a material such as aluminum or stainless steel. Thepower contained in the laser beam 14 pulse focused on target 18 shouldbe sufficient to cause a plasma to be created on target 18. The plasmain turn will generate soft X-rays in the range of two to twentyangstroms depending upon the material of target 18.

Target 18 must be in an evacuated chamber 24 which may be maintained ata pressure of 0.01 Torr by vacuum pump 26. Lens 16 may be positionednear the outer surface of chamber 24. An X-ray window 28 is alsoprovided in chamber 24 to allow the X-rays 22 to travel from theinterior of chamber 24 to the exterior thereof. Window 28 may be asuitable solid material, such as Beryllium, which is transparent toX-rays or it may be an aerodynamic jet stream such as described inco-pending U.S. patent application Ser. No. 669,442, filed Nov. 8, 1984,entitled "X-ray Lithography System" in the name of James Forsyth.

External to chamber 24 is positioned an X-ray mask for providing animage of X-rays to a semiconductor wafer 32 covered by X-ray resistmaterial 34. Wafer handler means 36 is provided to move the wafer tovarious positions so that different areas may be exposed to the patternof X-rays defined by mask 30. Such wafer handling equipment is shown inU.S. Pat. No. 4,444,492 in the name of Martin E. Lee and entitled"Apparatus for Projecting a Series of Images onto Dies of aSemiconductor Wafer".

System 10, as heretofore described is typical of the prior art taught bySmith et al., or Nagel et al. System 10 operates by a single pulse 14from laser 12 being provided and focused by lens 16 at a spot 38 oftarget 18. The focused laser pulse creates a plasma 20 which emitsX-rays 22. The X-rays travel throughout chamber 24 and in particularthrough window 28 and mask 30 to cause a pattern to be exposed onphotoresist 34. In the prior art, target 18 is either manually moved orreplaced after each laser pulse is applied thereto and the sameprocedure repeated. Due to the long length of time required to manuallymove or replace target 18, the practicalities of using the prior artversions of system 10 are quite limited. Conventional lithographyequipment, such as the photolithography systems described in theaforementioned U.S. Pat. No. 4,444,492, include wafer handling means 36which allow exposures to occur approximately every two seconds. Thus,the long time required to manually move or replace target 18 can nottake advantage of state of the art wafer handling means 36.

The teachings of the prior art also suggest that it is improper toprovide more than one pulse from laser 12 towards the same spot 38 ontarget 18. Each time a pulse is applied and a plasma created, a certainamount of material of target 18 is burnt away due to the tremendous heatof the plasma. This results in a cavity being formed on the surface oftarget 18 at point 38 where laser beam 14 strikes target 18. In the pastwhere experimenters have tried to direct a second laser beam into theformed cavity, the determination was that a much lower intensity ofX-rays were formed. In these experiments the detector for X-rayintensity was placed at an angle equal to or greater than 45 degreesaway from the path of beam 14.

Referring now to FIGS. 2A, 2B, 2C and 2D an explanation of what occurswhen the series of laser beam pulses are applied to target 18 will begiven. In FIG. 2A laser beam 14 strikes target 18 at point 38 causing aplasma 20 to be created and X-rays 22 to be emitted from plasma 20. Asseen in FIG. 2A, plasma 20 is created essentially on the outer surfaceof target 18. The X-rays 22 are provided with an approximate equalintensity throughout the entire area below target 18. After the plasma20 has dissipated in FIG. 2A a cavity 40 is formed, as shown in FIG. 2B.The cross sectional area of cavity 40 is larger than the cross sectionalarea of laser beam 14 at the point beam 14 strikes target 18 and theresulting cavity 40 has a smaller cross sectional area than does theinitial plasma, as seen in FIG. 2A. As additional laser pulses 14 areapplied against target 18 and focused on spot 38, cavity 40 increases indepth as shown in FIGS. 2C and 2D. As cavity 40 is formed and increasesin depth, plasma 20 is created within the cavity 40 until at point, asshown in FIGS. 2C and 2D, plasma 20 is wholly within cavity 40. Laserpulses could continue to be focused on point 38 and cavity 40 wouldbecome deeper as shown by the dashed lines in FIG. 2D.

As cavity 40 increases in depth and plasma 20 is formed within thecavity, the temperature of plasma 40 increases due to the more confinedarea in which it is formed. This causes a greater intensity of X-rays tobe emitted. In addition, these X-rays are confined to a certain conesurrounding laser beam 14 and represented by the conical angle 42 asshown in FIGS. 2B, 2C and 2D. Thus, the intensity of the X-rays withinthe cone defined by angle 42 is significantly greater than theintensities beyond the cone. In fact, the intensity of the X-rays beyondthe cone defined by angle 42 is significantly less than the intensity ofthe X-rays formed in FIG. 2A. This phenomena of a significantly higherintensity of X-rays within the cone defined by angle 42 escapeddetection in the prior art experimentations because of positioning ofthe X-ray detector outside of the cone defined by angle 42.

As the depth of cavity 40 increases, the angle of the cone defined byangle 42 decreases. X-rays are absorbed into the side of the cavitywall, thereby creating additional heat. The higher plasma density causedby the confinement of the cavity walls causes the plasma in the cavityto act more as an ideal radiator, thereby increasing the intensity ofthe X-rays.

Experiments have shown that cavities which produce significant increasesin X-ray output will be deep enough below the point 38 to cause angle 42to be approximately 45 degrees. If the cavity is formed by aprior/focused laser pulse, successive laser pulses focused into thecavity will cause the cavity to become deeper, reducing the availablecone angle 42. The intensity of the X-rays within the cone defined byangle 42 may be two or more times greater than the intensity of theX-rays formed, for instance, as shown in FIG. 2A. Repeated irradiationof the cavity by many laser pulses eventually causes the X-ray emissionwithin the cone angle 42 to decline. This occurs because the outer wallsof the cavity serve to act as a heat sink for the hot plasma created atthe bottom of the cavity. This colder material acts to absorb some ofthe X-ray radiation under these conditions. Empirically, a range ofcavity depths produced by one or more focused laser pulses of from 0.3to 0.8 mm is found to produce significantly enhanced X-ray emissionwithin cone angle 42.

Referring again to FIG. 1, in order to take advantage of the phenomenaof the higher intensity X-rays within the cone defined by angle 42,laser 12 and lens 16 are positioned so that the window 28, mask 30 andthe portion of semiconductor wafer 32 being exposed are all positionedwithin the critical angle 42 of the cone of high intensity X-rays. Thisrequires that lens 16 be placed in close proximity to window 28. Thismay require the use of directing mirrors so that laser 12 does notinterfere with the wafer handling device 36 as it moves wafer 34 betweenthe various positions to be exposed.

By placing mask 30 and the area of wafer 32 which is to be exposedwithin the cone defined by angle 42, significant advantages areachieved. First a lower powered laser 12 may be used to generate thenecessary intensity of X-rays to expose the X-ray resist 34. Further,with a higher intensity of X-rays, different types of X-ray resistmaterial may be used. Further, the ability to direct a plurality oflaser pulses at the same point allows multiple exposures to be obtainedwithout having to move or replace target 18.

Referring now to FIG. 3, a different configuration target is shown whichcan be mechanically moved so that a large quantity of X-ray exposurescan be easily and automatically obtained without having to physicallychange the target. In FIG. 3, where like components to FIG. 1 are shown,they are given like numeric reference numbers. In FIG. 3, the X-raylithography system 44 includes a mirror 46 for directing the laser beam14 from laser 12 through lens 16. The other difference is that target 18shown in FIG. 1 is replaced by target system 48. Target system 48includes a cylindrical drum 50 constructed of an appropriate materialuseful as a target. Such material may be aluminum or stainless steel aspreviously mentioned. Attached to the sides of cylinder 50 is a shaft 52and a threaded rod 54. A motor 56 is attached to shaft 52 for rotatingshaft 52, which in turn rotates drum 50. Motor 56 rides on a rail 58.Threaded rod 54, which is attached to the other side of cylinder 50, isapplied into a threaded chamber 60. As motor 56 turns shaft 52 andcylindrical drum 50, rod 54 moves into chamber 60, thereby causinglateral movement of the entire target system 48. During this movementmotor 56 rides along rails 58.

In operation, cylindrical drum 50 is positioned so that laser beam 14 isapplied initially at one end thereof. After an appropriate number oflaser beam 14 pulses have been applied to a particular spot on drum 50,drum 50 is rotated a small amount. This allows a new series of pulses tobe applied to a point just adjacent to the prior point to which thelaser pulses were previously applied. As drum 50 continues to be rotateda small amount each time, a helical path of target points is formed. Theamount in which drum 50 is rotated is determined by the diameter of thecavity 40 shown in FIGS. 2B, 2C and 2D. As cavity 40 is a very smallsize, in the order of 0.5 mm or less, the number of cavities permittedper rotation of drum 50 is great. Of course the size of drum 50 willdetermine the actual number of available spots to create cavities.Further the lateral movement per rotation will be quite small. Theactual size of drum 50 can be selected so that a large number of targetpositions can be obtained. This will allow the target to be used for along period of time before replacement is necessary.

Referring now to FIG. 4, an alternate target system 62 is shown whichlikewise will provide a large number of target areas. Target system 62includes a long target strip 64, which initially is totally wound arounda feed reel 66 and threaded to a take-up reel 68. Take-up reel 68 isdriven by a motor 70 to cause strip 66 to move in short increments pastpoint 38 at which laser beam 14 is focused. Target system 62 may befabricated into a cassette 74 for easy removal and replacement. Lateralmovement of the cassette 74 may also be provided to increase the numberof target spots available. In the alternative means associated withmirror 46 could direct the laser beam 14 pulses at selected locationsperpendicular to the path of movement of strip 64.

In using both long life target systems 48, shown in FIG. 3, and 62 shownin FIG. 4, it is necessary to provide the initial cavity 40 on thetarget. This may be accomplished by providing an initial laser pulse toeither drum 50 or strip 64 while blocking the X-rays provided throughwindow 28 from reaching mask 30. Such blockage may be accomplished byproviding a shutter 72 within window 28. Shutter 72 may be an X-rayabsorbing material strobed across window 28 in conjunction with theformation of cavity 40. In the alternative, a higher energy laser beampulse could be provided initially to cause a greater intensity of X-raysthroughout the entire area of chamber 24. Thereafter, lower energy laserpulses 14 would be provided. By controlling the energy within each ofthe laser pulses 14 provided against the target 50 or 64, the intensityof the X-rays passing through window 28 can be controlled. Further, aninitialization procedure could be devised where the initial cavities areall created before the chip handling mean 36 moves the chip 32 intoposition. If such a procedure were utilized, means would be required toinitialize the target position so that after initialization, laserpulses would be directed into the performed cavities.

What I claim is:
 1. An x-ray generating system comprising:means forproviding an excitation beam along a path; a target having a cavitytherein, said target being positioned so that said cavity intersectssaid path, said beam being of sufficient power to cause an X-rayemitting plasma to be created at least partially within said cavity; andan object to be irradiated by said emitted X-rays positioned within acertain angle of said path, said certain angle being that angle withrespect to said path within which the intensity of the provided X-raysis significantly greater than the intensity of the X-rays provided if apart of said target, other than said cavity, was positioned to intersectsaid path.
 2. The invention according to claim 1 wherein said cavity isformed by said excitation beam being directed towards a noncavity areaof said target.
 3. The invention according to claim 1 wherein saidexcitation beam is a laser pulse.
 4. The invention according to claim 1wherein said cavity has a smaller cross sectional area than the crosssectional area of the plasma created when said beam strikes a noncavityarea of said target.
 5. The invention according to claim 4 wherein saidcavity has cross sectional area larger than the cross sectional area ofsaid beam at the point said beam strikes said cavity.
 6. The inventionaccording to claim 5 wherein said excitation beam is a laser pulse. 7.The invention according to claim 1 wherein said cavity has across-sectional area larger than the cross sectional area of said beamat the point said beam strikes said cavity.
 8. The invention accordingto claim 7 wherein said cavity is formed by said excitation beam beingdirected towards a noncavity area of said target.
 9. The inventionaccording to claim 1 wherein said beam is directed towards said cavity apluality of times.
 10. The invention according to claim 9 wherein saidcavity increases in depth after each time said beam is directed towardssaid cavity.
 11. The invention according to claim 10 wherein said cavityis formed by said excitation beam being directed towards a noncavityarea of said target.
 12. The invention according to claim 11:wherein theintensity of the X-rays emitted towards said object during the initialformation of said cavity is less than the intensity of the X-raysemitted thereafter; and wherein said system further comprises means toprevent the X-rays emitted during formation of the cavity from strikingsaid object.
 13. The invention according to claim 1:wherein said objectis at least one desired area on a semiconductor substrate; wherein saidcavity is formed by said beam being directed towards a noncavity area ofsaid target; and wherein said system further comprises means to preventX-rays generated during the formation of said cavity from irradiatingsaid desired area of said substrate.
 14. In an X-ray generating systemhaving excitation beam generating means, target means, and an object tobe irradiated with X-rays, said excitation beam being directed tointersect a surface of said target means with sufficient power to createan X-ray plasma at the point said beam intersects said target means,said X-rays being emitted towards said object, the improvementcomprising:a cavity on the surface of said target means; means forcausing said beam to intersect said cavity; and means for positioningsaid object within a critical angle of said beam in which the intensityof the X-rays emitted exceeds the intensity of the X-rays emitted whensaid beam intersects an area of said target other than a cavity.
 15. Theinvention according to claim 14 wherein said target is metal.
 16. Theinvention according to claim 15 wherein said object is a semiconductorsubstrate.
 17. The invention according to claim 16 wherein said targetis positioned within an evacuated chamber.
 18. The invention accordingto claim 14 wherein said cavity is formed by said beam being directedtowards an area of said target not having a cavity, said beam thereafterbeing directed into said formed cavity.
 19. The invention according toclaim 18 wherein said beam is repeatedly directed into said cavity aplurality of times.
 20. An X-ray lithography system comprising:a highrepetition rate, high power laser pulse generator; a target towardswhich said laser pulse is directed, said target being of a material inwhich a soft X-ray emitting plasma is created each time a laser pulse isdirected towards said target, said target having a cavity therein formedby at least one prior laser pulse having been directed towards saidtarget and positioned so that at least one subsequent laser pulse isdirected into said cavity to create at least a portion of said plasma insaid cavity, so that a region significantly of higher intensity X-raysresults; and semiconductor wafer holding and moving means forpositioning selected areas of a semiconductor wafer in an exposing area,said exposing are being within said significantly higher intensity X-rayregion resulting from said plasma being formed at least partially withinsaid cavity.